Citation:

Abstract:

Coherence based source analysis techniques can be used to identify the
contribution of combustion noise in the exhaust of a jet engine and hence
enable the design of noise reduction devices. However, when the combustion
noise propagates in a non-linear fashion the identified contribution using
ordinary coherence methods will be inaccurate. In this paper, an analysis
technique to enable the contribution of linear and non-linear mechanisms
to the propagated sound to be identified is reported. An experimental rig
to study the propagation of noise through a rotor/stator set-up using a
vane-axial fan mounted in a duct so that non-linear interactions between a
sound source and the fan could be investigated is described. The technique
which is used to identify a non-linear tone generated by the interaction of
the rotor and a propagated tone is reported. The identification procedures
are then applied to data from full scale turbo-fan engine tests instrumented
with pressure transducers at the combustor can and in the hot-jet pipe with
microphones in the near-field. At a particular power setting, the interaction
between the combustion noise and the high pressure turbine was measured
in the hot jet pipe. The analysis techniques enabled non-linear interactions
to be identified and linear and non-linear coherent output powers to be
determined.